* vm_exec.h, tool/instruction.rb: not an error, but a BUG if stack
  overflow checking failed just before/after the beginning of an
  instruction. It should be treated as a BUG.
  Please tell us if your code cause BUG with this problem.
  This check will removed soon (for performance).



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42194 b2dd03c8-39d4-4d8f-98ff-823fe69b080e